Variable capacitance bridge accelerometer

ABSTRACT

A bridge accelerometer system includes four capacitors, wherein two capacitors are formed on each side of a rigid member. The other two capacitors are similarly constructed, except that the rigid member is replaced by a flexured plate. The construction of the plates with respect to the flexured plate is substantially similar to the configuration formed by the rigid member and the other two capacitors, and the fixed capacitors and rigid plate are isolated from the flexured arrangement. The four capacitors are connected to form a bridge generating a bridge voltage signal. The bridge voltage signal is signal processed to generate a digital word. The digital word is linearized and filtered and received in computer for attitude adjustment and velocity control.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present invention is related to application (Attorney Docket 02-0998) entitled “Flexure Plate Dual Capacitance Accelerometer,” filed on Jan. 16, 2003 and incorporated by reference herein.

TECHNICAL FIELD

[0002] The present invention relates generally to accelerometers, and more particularly, to a variable capacitance bridge accelerometer.

BACKGROUND ART

[0003] It is well known that capacitive accelerometers measure the acceleration, vibration and the inclination of objects to which they are attached. These objects typically include missiles, spacecraft, airplanes and automobiles.

[0004] In general, capacitive accelerometers change electrical capacitance in response to acceleration forces and vary the output of an energized circuit. Capacitive accelerometer systems generally include sensing elements, including capacitors, oscillators, and detection circuits.

[0005] The sensing elements include at least two parallel plate capacitors functioning in differential modes. The parallel plate capacitors generally operate in sensing circuits and alter the peak voltage generated by oscillators when the attached object undergoes acceleration.

[0006] When subject to a fixed or constant acceleration, the capacitance value is also a constant, resulting in a measurement signal proportional to uniform acceleration.

[0007] This type of accelerometer can be used in an aerospace or in a portion of aircraft or spacecraft navigation or guidance systems. Accordingly, the temperature in the operating environment of the accelerometer changes over a wide range. Consequently, acceleration must be measured with a high accuracy over a wide range of temperatures and temperature gradients. This is often a difficult and inefficient process.

[0008] The disadvantages associated with current accelerometer systems have made it apparent that a new accelerometer is needed. The new accelerometer should substantially minimize temperature sensing requirements and should also improve acceleration detection accuracy. The present invention is directed to these ends.

SUMMARY OF THE INVENTION

[0009] In accordance with one aspect of the present invention, an accelerometer includes a flexure plate section and a rigid plate section enclosed within a housing. The flexure plate section includes a first fixed plate, a second fixed plate spaced apart from and in parallel relation to the first plate, and a flexure plate disposed between and in substantially parallel relation to the first and second plates. The flexure plate is coupled to the housing along at least an edge. The flexure plate and first plate define a first distance and a first capacitor and the flexure plate and the second plate define a second distance and a second capacitor, wherein the first and second distances vary in response to acceleration forces acting upon the flexure plate. The rigid plate section is enclosed within the housing and electrically isolated from the flexure plate section. The rigid plate section includes a third fixed plate, a fourth fixed plate spaced apart from and in parallel relation to the first plate, and a rigid plate disposed between and in substantially parallel relation to the first and second plates. The rigid plate is electrically isolated from the housing. The rigid plate and the third plate define a third distance and a third capacitor and the rigid plate and the fourth plate define a fourth distance and a fourth capacitor, wherein a fifth distance is defined between the rigid plate and the flexure plate such that the fifth distance is a function of the acceleration forces. The first, second, third, and fourth capacitors connect to form a bridge adapted to generate a bridge voltage signal.

[0010] In accordance with another aspect of the present invention, a method for operating an accelerometer system includes accelerating a flexure plate, thereby causing a first distance between the flexure plate and a first fixed plate to change and thereby causing a second distance between the flexure plate and a second fixed plate to change. A bridge waveform is then generated and rectified as a function of a reference excitation phase. A rectified waveform is generated and filtered. An analog signal is generated and converted to a digital signal. A digital word is generated.

[0011] One advantage of the present invention is that it generates a dynamic range and granularity sufficient for Inter-Continental Ballistic Missile (ICBM) usage. Moreover, the bridge accelerometer consumes less power than current accelerometers, while dramatically improving reliability.

[0012] Another advantage is that it is not substantially affected by changes in temperature or temperature gradients. While the bridge configuration reduces the temperature sensitivity, the AC excitation allows narrow band analog filtering. These effects enhance the signal-to-noise ratio.

[0013] Additional advantages and features of the present invention will become apparent from the description that follows, and may be realized by means of the instrumentalities and combinations particularly pointed out in the appended claims, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order that the invention may be well understood, there will now be described some embodiments thereof, given by way of example, reference being made to the accompanying drawings, in which:

[0015]FIG. 1 illustrates an aerospace system including an accelerometer system in accordance with one embodiment of the present invention;

[0016]FIG. 2 illustrates the accelerometer system of FIG. 1;

[0017]FIG. 3 illustrates the equivalent circuit for the capacitors of FIG. 2; and

[0018]FIG. 4 illustrates a logic flow diagram of the aerospace system of FIG. 1 in operation, in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention is illustrated with respect to a bridge accelerometer, particularly suited to the aerospace field. The present invention is, however, applicable to various other uses that may require accelerometers, such as any system requiring position and velocity measurements under extreme conditions, as will be understood by one skilled in the art.

[0020] Referring to FIGS. 1 and 2, the missile or aerospace system 10, including a bridge accelerometer 12 within an inertial measurement unit, is illustrated. The aerospace system 10 is merely an illustrative example of an accelerating object and not meant to be limiting. For example, the present bridge accelerometer 12 could be implemented in any accelerating object to sense acceleration forces, including any type of vehicle or missile system, such as a Minuteman III missile system or a Scud missile system.

[0021] The illustrated aerospace system 10 includes an inertial measurement unit 13 including three bridge accelerometers 12, 15, 17 and a serial data bus 18. The three accelerometers, the x-axis accelerometer 12, the y-axis accelerometer 15, and the z-axis accelerometer 17, are coupled to gimbals and gimbal torque motors 20 (yaw, pitch and roll motors). The accelerometers 12, 15, 17 are also coupled to the serial bus 18, which transfers information to a computer/processor 14. The computer 14 is coupled to the missile steering nozzle (or vane actuators) unit 16 and the gimbal torque motors 20.

[0022] The bridge accelerometer 12 or variable capacitance bridge accelerometer (VCBA) is a single axis accelerometer that generates a robust wide dynamic range of performance. Important to note is that alternate embodiments of the present invention have one or more accelerometers, the three illustrated accelerometers 12, 15, 17 are only one example of a possible arrangement of accelerometers, and any number of accelerometers can be utilized.

[0023] The accelerometer 12 will be described as an illustrative example of the three accelerometers 12, 15, 17 in this embodiment. The accelerometer 12 is part of the inertial measurement unit 13 and includes a housing 36, a flexured plate section 22, a rigid plate section 24, a ground 38, an AC source 40, a differential amplifier 42, a demodulator 44, an analog filter 46, an analog-to-digital converter 48, and a digital linearizer and filter 50.

[0024] The housing 36 or metal housing structure encloses four capacitors, which will be discussed later. A gas or vacuum environment is also enclosed therein such that there is no interference with the movement of the flexure plate 30 other than the acceleration of the system 10 along a perpendicular axis.

[0025] The flexured plate section 22 includes a single flexure plate 30 and two parallel fixed plates 32, 34. The rigid plate section 24 includes a rigid plate and two fixed plates. The two sections are electrically isolated and enclosed in a metal housing structure 36.

[0026] In the present embodiment, the flexure plate 30 is coupled to the housing 36 at only one edge 37. Numerous other attachment points are, however, included, as will be understood by one skilled in the art. The flexure plate includes a first side 31, a second side 33 and a common edge 37.

[0027] The flexure plate 30 is positioned between the first and second fixed plates 32, 34 such that the first fixed 32 plate is a first distance (d₁) from the first side 31 and the second fixed plate 34 is a second distance (d₂) from the second side 33 of the flexure plate 30. The flexure plate 30 is affixed to the metal housing structure 36 through at least a portion 35 of the common edge 37 of the flexure plate 30, which is also coupled to a ground 38.

[0028] The flexure plate is rigidly fixed to the metal housing structure 36 through almost any manner known in the art. Resultantly, all the system flexure is generated within the flexure plate 30. This generally increases reliability and robustness of the system 10. This, however, generates a non-linear output from the flexure plate 30, which will be discussed regarding the linearizer 50.

[0029] The combination of the first fixed plate 32 and the flexure plate 30 forms a first parallel plate capacitor, and the combination of the second fixed plate 34 and the flexure plate 30 forms the second parallel plate capacitor. The equivalent capacitor for the first parallel plate capacitor is illustrated in FIG. 3 in broken lines as C₁, and the equivalent capacitor for the second parallel plate capacitor is illustrated in broken lines as C₂.

[0030] The capacitance of the parallel plate capacitors is determined by the following: C≅(ε₀A)/d, where ε₀ is the permittivity constant, A is the area of a fixed plate 32 or 34, and d is the effective distance between the flexure plate 30 and one of the fixed plates 32, 34.

[0031] The first fixed plate 32 is coupled to the metal housing structure 36 and positioned a first distance (d₁) from the flexure plate 30. The first fixed plate 32 and the flexure plate 30 form a first capacitor whose operation is also governed by the equation C≅(ε₀A)/d. The first fixed plate 32 responds to movement of the flexure plate 30 when d₁ either increases or decreases, thereby generating a first phase shift capacitance signal.

[0032] The second fixed plate 34 is also coupled to the metal housing structure 36 and positioned a second distance (d₂) from the flexure plate 30. The second fixed plate 34 and the flexure plate 30 form a second capacitor whose operation is governed by the equation C≅(ε₀A)/d. The second fixed plate 34 responds to movement of the flexure plate 30 when d₂ either increases or decreases, thereby generating a second phase shift capacitance signal.

[0033] The distances (d₁ and d₂) between the flexure plate 30 and the fixed plates 32, 34 are a function of acceleration and are proportional or equal when the system 10 is at rest.

[0034] During acceleration, the flexure plate 30 flexes according to the reaction force of Newton's second law of motion, force=mass×acceleration (F=ma), causing the distance between the flexure plate 30 and the fixed plates 32, 34 to vary, thus creating the two variable capacitors C₁, C₂, one on each side of the flexure plate 30.

[0035] For the rigid plate section 24, which is insulated from the flexured plate section 22, the rigid plate 60 is positioned between the third fixed plate 62 and fourth fixed plate 64 such that the third fixed plate 62 is a third distance (d₃) from a first side 66 and the fourth fixed plate 64 is a fourth distance (d₄) from a second side 68 of the rigid plate 60. The rigid plate 60 is coupled to an insulator 70 through at least a portion of at least one common edge 72 of the first side 66 and the second side 68 of the rigid plate 60, and the insulator 70 is affixed to the metal housing structure 36. The third and fourth fixed plates 62, 64 are coupled to the housing 36.

[0036] In the present embodiment, the rigid plate 60 is coupled to the housing 36 through an insulator at only one edge 72. However, numerous other attachment points are included, as will be understood by one skilled in the art.

[0037] The combination of the third fixed plate 62 and the rigid plate 60 forms a third parallel plate capacitor, and the combination of the fourth fixed plate 64 and the rigid plate 60 forms the fourth parallel plate capacitor. The equivalent capacitor for the third parallel plate capacitor is illustrated in broken lines in FIG. 3 as C₃, and the equivalent capacitor for the forth parallel plate capacitor is illustrated in broken lines as C₄.

[0038] The first and second capacitors are formed on each side of the flexure plate 30 and the third and fourth capacitors are formed on either side of the rigid plate 60. The four capacitors are electrically connected to form a bridge. The fixed capacitors (third and fourth) and rigid plate 60 are isolated from the flexured plate 30 and flexured plate capacitors (first and second). All capacitors are designed to be as nearly equal as possible when at rest.

[0039] The distance between the flexure plate 30 and the fixed plate 60 is a function of acceleration. The center of each bridge side (A and C in FIGS. 2 and 3) is monitored to detect the differential amplitude. As the flexure plate 30 flexes in response to acceleration, one capacitor increases and the other decreases, thereby increasing the bridge voltage on one side and decreasing bridge voltage on the other.

[0040] The bridge is excited with an AC source 40 at one end and grounded at the other. The ground 38 is coupled to the flexure plate 30 and the AC source 40 is coupled to the rigid plate 60. The two capacitive legs (E, D) and (F, B) of the bridge produce two voltage dividers, each of which provides a terminal (A, C), illustrated in FIG. 3, to measure the resulting voltage.

[0041] The bridge configuration reduces the temperature sensitivity and the AC excitation allowing narrow band analog filtering, both of which enhance the signal-to-noise ratio. The bridge circuitry utilizes GaAs or high speed CMOS, as the accuracy required for performance will require low propagation delays. In one embodiment, the bridge circuitry is mounted on a heated housing structure. In addition, the entire system includes a precision heating device 43 and sufficient mass to reduce gradients in the bridge in one embodiment.

[0042] The voltage phase gives direct indication of the direction of acceleration. This output is gain adjusted if required in the differential amplifier 42, and received in the demodulator 44, which rectifies the waveform as a function of the reference excitation phase from the AC source 40. The resulting waveform is then filtered in the analog domain in the analog filter 46 and received in an analog-to-digital converter 48 where the data becomes a digital word.

[0043] The digital word is then filtered and linearized in the digital linearizer and filter 50 for manufacturing and flexure non-uniformities. This output is a digital word having a magnitude proportional to the acceleration of the system in either direction along the perpendicular axis.

[0044] In other words, the linearizer 50 receives the overall digital word signal. The linearizer 50 compensates for both the nonlinear function generated from the analog-to-digital converter 48 and any manufacturing anomalies, as will be understood by one skilled in the art. The linearizer 50 value is established in manufacturing through taking large samples of performance curves, as will be understood by one skilled in the art. The linearizer 50 output is a digital word whose magnitude is proportional to the acceleration of the system 10 in either direction along an axis perpendicular to the flexure plate 30.

[0045] Numerous alternate linearizers are also included in the present embodiment whereby a substantially linear function can be generated by compensating for nonlinear functions, for example, in the digital domain, a digital linearizer is included. The output of the linearizer 50 is an acceleration signal multiplied by a constant (k).

[0046] Statistical filtering of the linearized data somewhere significantly above the maximum flexure frequency also occurs in either the digital linearizer and filter 50 or the computer 14 to reduce the overall noise impact on the system 10. The compensation for the non-linearity of the flexure structure and overall transport error is compensated for by the linearizer and filter 50 whose values are established in manufacturing through sampling performance curves.

[0047] The computer 14 receives the acceleration signal multiplied by the constant and generates a computer signal and response thereto. The computer 14 is embodied as a typical missile or airplane computer, as is familiar in the art.

[0048] The missile steering nozzle or vane actuators 16 receive the computer signal and activate the gimbal torque motors 20 or object control devices in response thereto.

[0049] Referring to FIG. 4, a logic flow diagram 100 illustrating a method for acceleration control is illustrated. Logic starts in operation block 102 where power is applied to the system, the missile platform is aligned and the capacitive accelerometer is activated.

[0050] In operation block 104, strategic alert biasing occurs and sensor data is compared to a known reference.

[0051] In operation block 106, the missile is launched.

[0052] In operation block 108, the missile accelerates and the flexure plate flexes to either increase or decrease d₁ or d₂ thereby causing the bridge voltage on one side to increase while decreasing on the other side. These voltages are received in the differential amplifier and gain adjusted. The gain adjusted signal is rectified in the demodulator as a function of the excitation phase from the AC source. The phase indicates the direction of the acceleration. The adjusted gain signal is filtered in the analog domain in an analog filter and converted to a digital word in the analog-to-digital converter. Data from the accelerometer(s) is processed by the missile computer or attitude controller.

[0053] In operation block 110, missile systems respond to the acceleration. In other words, the computer receives a signal indicating that acceleration of the system has changed. In response to this change, for example, thrusters are activated to compensate for the acceleration change. In other words, the missile computer/controller/processor controls the flight profile through the missile nozzle or steering vane actuators.

[0054] In operation, a method for operating a bridge accelerometer system includes a method for operating an accelerometer system includes accelerating a flexure plate, thereby causing a first distance between the flexure plate and a first fixed plate to change and thereby causing a second distance between the flexure plate and a second fixed plate to change. A bridge waveform is then generated and rectified as a function of a reference excitation phase. A rectified waveform is generated and filtered. An analog signal is generated and converted to a digital signal, and a digital word is generated. The digital word is linearized and filtered and received in the computer for attitude adjustment and velocity control.

[0055] From the foregoing, it can be seen that there has been brought to the art a new and improved accelerometer system. It is to be understood that the preceding description of the preferred embodiment is merely illustrative of some of the many specific embodiments that represent applications of the principles of the present invention. For example, a vehicle, such as an airplane, spacecraft, or automobile could include the present invention for acceleration detection and control. Numerous and other arrangements would be evident to those skilled in the art without departing from the scope of the invention as defined by the following claims. 

What is claimed is:
 1. An accelerometer comprising: a housing; a flexure plate section enclosed within said housing and comprising a first fixed plate, a second fixed plate spaced apart from and in parallel relation to said first plate, and a flexure plate disposed between and in substantially parallel relation to said first and second plates, said flexure plate coupled to said housing along at least an edge, said flexure plate and first plate defining a first distance and a first capacitor and said flexure plate and said second plate defining a second distance and a second capacitor, wherein said first and second distances vary in response to acceleration forces acting upon said flexure plate; and a rigid plate section enclosed within said housing and electrically isolated from said flexure plate section and comprising a third fixed plate, a fourth fixed plate spaced apart from and in parallel relation to said first plate, and a rigid plate disposed between and in substantially parallel relation to said first and second plates, said rigid plate electrically isolated from said housing, said rigid plate and said third plate defining a third distance and a third capacitor and said rigid plate and said fourth plate defining a fourth distance and a fourth capacitor, wherein a fifth distance is defined between said rigid plate and said flexure plate such that said fifth distance is a function of said acceleration forces, said first capacitor, said second capacitor, said third capacitor and said fourth capacitor connect to form a bridge adapted to generate a bridge voltage signal.
 2. The system of claim 1 further comprising: a differential amplifier adapted to adjust a gain of said bridge voltage signal.
 3. The system of claim 1 further comprising: demodulator adapted to receive said bridge voltage signal and rectify said bridge voltage signal as a function of a reference excitation phase thereby generating a rectified waveform.
 4. The system of claim 3 further comprising: an analog filter adapted to receive said rectified waveform and generate a filtered analog signal therefrom.
 5. The system of claim 4 further comprising: an analog-to-digital converter adapted to receive said filtered analog signal and generate a digital word therefrom
 6. The system of claim 5 further comprising: a digital linearizer adapted to receive said digital word and generate a linearized signal therefrom.
 7. The system of claim 6 further comprising: a computer adapted to activate a system component in response to said linearized signal.
 8. A method for operating an accelerometer system comprising: accelerating a flexure plate, thereby causing a first distance between the flexure plate and a first fixed plate to change and thereby causing a second distance between the flexure plate and a second fixed plate to change; generating a bridge waveform; rectifying said bridge waveform as a function of a reference excitation phase; generating a rectified waveform; filtering said rectified waveform; generating an analog signal; converting said analog signal to a digital signal; and generating a digital word.
 9. The method of claim 8 further comprising: adjusting a gain of said bridge waveform.
 10. The method of claim 8 further comprising: linearizing said digital word.
 11. The method of claim 8 further comprising: activating an object control device in response to said digital word.
 12. An accelerometer system comprising: a demodulator adapted to receive a bridge voltage signal and rectify said bridge voltage signal as a function of a reference excitation phase thereby generating a rectified waveform; an analog filter adapted to receive said rectified waveform and generate a filtered analog signal; an analog-to-digital converter adapted to receive said filtered analog signal and generate a digital word therefrom; a flexure plate, wherein a first capacitor is formed on a first side of said flexure plate and a second capacitor is formed on a second side of said flexure plate; and a rigid plate electrically isolated from said flexure plate, wherein a third capacitor is formed on a first side of said rigid member and a fourth capacitor is formed on a fourth side of said rigid plate, wherein said first capacitor, said second capacitor, said third capacitor and said fourth capacitor are connected to form a bridge, said bridge grounded at a first terminal and excited by a power source adapted to generate said reference excitation phase at a second terminal and adapted to generate said bridge voltage signal between said first terminal and said second terminal in response to acceleration forces acting on said flexure plate.
 13. The system of claim 12 further comprising: a differential amplifier adapted to adjust a gain of said bridge voltage signal.
 14. The system of claim 12 further comprising: a linearizer adapted to linearize said digital word.
 15. The system of claim 14 wherein said bridge, said demodulator, said analog filter, said analog-to-digital converter, and said linearizer are included in a first accelerometer.
 16. The system of claim 15 further comprising: a second accelerometer and a third accelerometer, wherein said second and third accelerometers are arranged with said first accelerometer to receive cross axis thrust data.
 17. The system of claim 16 further comprising: a serial data bus adapted to receive acceleration signals from said first, second and third accelerometers, said serial data bus further adapted to exchange information with a computer.
 18. The system of claim 17, wherein said first accelerometer is coupled to a yaw torque motor, said second accelerometer is coupled to a pitch torque motor, and said third accelerometer is coupled to a roll torque motor.
 19. The system of claim 12 further comprising: a housing enclosing said first capacitor, said second capacitor, said third capacitor and said fourth capacitor in a gas or vacuum environment.
 20. The system of claim 19 further comprising: a precision heating device coupled to said housing adapted to reduce gradients in said bridge. 